This invention relates to a wavefront measuring method in a projection exposure apparatus usable in a lithographic process, for the manufacture of semiconductor devices, for transferring a pattern of a mask onto a photosensitive substrate through a projection optical system. In another aspect, the invention is concerned with a projection exposure apparatus which uses such a method.
A photolithographic process for the manufacture of semiconductor devices, for example, uses a projection type exposure apparatus for transferring a circuit pattern, formed on a reticle or photomask (hereinafter xe2x80x9creticlexe2x80x9d), onto a semiconductor wafer, for example, being coated with a photosensitive material. In such an exposure apparatus, it is required that a pattern of a reticle is accurately transferred onto a wafer with a predetermined magnification (reduction ratio). It is, therefore, important to use a projection lens having a good imaging performance and reduced aberration. Particularly, in recent years, further miniaturization of a semiconductor device has been required more and more. Therefore, in many cases, a pattern corresponding to the limit imaging performance for a pattern transfer through a projection lens, has to be transferred. As a result, a pattern to be transferred becomes more sensitive to aberration of an optical system. In these situations, it is desired to measure and control the imaging performance of a projection lens. Many proposals have been made in regard to the measurement and control.
An example of measuring the imaging performance of a projection lens is a phase restoration method. The phase restriction method is used mainly to improve the resolution of an optical system such as an astronomical telescope, having a large aberration, or an electron microscope. In accordance with this method, a phase distribution of an image is detected on the basis of image intensity distributions at plural points such as a focal plane (surface), a pupil plane, a defocused position and the like, and then a wavefront aberration of the optical system is calculated from the detected phase distribution.
FIG. 1A shows an algorithm of an ordinary phase restoration method. First, an arbitrary phase is assigned to a measured intensity distribution at a focal plane and, then, through Fourier transform, a complex amplitude distribution upon a pupil plane is detected. Subsequently, while holding a phase portion of the thus detected complex amplitude distribution unchanged, only an absolute value corresponding to an intensity portion thereof is replaced by a value (the square root of the intensity on the pupil plane) corresponding to an actually measured value, by which a fresh complex amplitude distribution is determined. Then, by performing inverse Fourier transform to the thus determined complex amplitude distribution, a complex amplitude distribution on the focal plane is detected. Then, while holding again its phase portion as it is, the intensity is replaced by an actually measured value.
The above-described calculations are repeated until convergence, whereby complex amplitude distributions on the focal plane and the pupil plane are detected. From the phase distribution in the complex amplitude distribution upon the pupil plane, the wavefront aberration of a lens can be calculated.
In a case wherein measurement of an intensity distribution upon a pupil plane is difficult to accomplish, as in the case of photolithography, another method as reported in J.J.A.P. Vol. 36, 1997, pages 7494-7498 may be used. In accordance with this method, as shown in FIG. 1B, between a focal plane and a defocused plane, across a pupil plane, transform and inverse transform are repeated, by which a complex amplitude distribution on the focal plane and a complex amplitude distribution on the defocused plane are calculated. Then, from the results of the calculation, the phase distribution on the pupil plane, that is, the wavefront aberration of a projection optical system is detected.
Generally, in a projection exposure apparatus, direct measurement of an intensity distribution upon a pupil plane is difficult to accomplish. For this reason, in order to detect a wavefront aberration of a projection optical system (projection lens) corresponding to a phase distribution on a pupil plane thereof on the basis of the phase restoration method, it is necessary to measure light intensity distributions upon a focal plane and a defocused plane.
In the measurement of a light intensity distribution, not only the measurement time for measuring light intensity distributions on the focal plane and the defocused plane but also the movement time of a light intensity distribution measuring system as well as the calculation time for calculation of a wavefront aberration of a projection optical system, are all important factors. If any of these two time periods is long, it is difficult to accurately measure the aberration of the projection optical system which is variable slightly during an exposure process due to absorption of exposure light by the projection optical system.
It is an object of the present invention to provide a projection exposure apparatus having a function for measuring a wavefront aberration quickly and precisely, by which not only a dynamic state but also a dynamic imaging performance of a projection optical system, which is variable slightly due to an exposure process, can be detected in an arbitrary time period.
In accordance with an aspect of the present invention, there is provided a projection exposure apparatus or a wavefront aberration measuring method for a projection exposure apparatus, wherein a light intensity distribution upon a pupil plane, in a reference state of exposure, is detected beforehand. In one preferred form of this aspect of the present invention, a wavefront aberration of a projection exposure lens in an arbitrary state during an exposure process can be detected in accordance with a phase restoration method, while referring to a measured value of an image plane light intensity distribution in an arbitrary state as well as a pupil plane intensity distribution in the reference state. Here, the term xe2x80x9carbitrary statexe2x80x9d means the state at an arbitrary moment where the optical characteristic of the projection optical system at the reference state is changed due to an exposure load or an environmental change (e.g., temperature, pressure or humidity).
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.